In the dentistry, various dental restorative materials are used for restorating a deficit after medically treating caries of a natural tooth, etc. Among them, a porcelain tooth obtained by sintering a high-fusing dental porcelain material made of, as a main raw material, feldspar (hereinafter simply referred to as "dental porcelain material") at 1200.degree. C. to 1300.degree. C. is not only chemically stable but also has translucency and color tone well matched with a natural tooth and hence, it has hitherto been widely used.
However, when such a porcelain tooth is set and fixed in the mouth and is applied with external forces by, for example, mastication, etc., it often breaks. In order to solve such a problem, various attempts to increase the strength of the porcelain tooth have been made but not yet reached satisfactory effects. For example, an attempt to provide a porcelain tooth by adding a crystal of high purity alumina to a dental porcelain material and then firing had the following problems. Namely, in order that the porcelain tooth reproduces color tone as in a natural tooth, the porcelain tooth must be prepared by firing several porcelain materials having different colors in the multilayered form. In this case, if the alumina crystal is present in the outer layer thereof, the translucency and color tone as in a natural tooth are lost. Accordingly, nevertheless it is desired to particularly reinforce the outer layer because it is applied directly with an external force, the alumina crystal cannot be present in any other layers than the inner layer. Such a porcelain tooth in which a crystal of alumina is preset only in the inner layer is slightly improved in the strength, which is not yet satisfactory, however. Under such a background, development of a method for thoroughly strengthening the porcelain tooth while maintaining translucency and color tone as in a natural tooth has been demanded in the field of dental restoration.
In order to meet such a demand, the inventors have extensively investigated and finally found that when after firing, a porcelain tooth is deposited with a specified inorganic salt of metal and then heated at a specified temperature, the porcelain tooth can be thoroughly strengthened in the non-molten state of the inorganic salt, thereby achieved this invention.